Author Topic: I designed and built a two-port vector network analyzer  (Read 523 times)

Offline profdc9

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I designed and built a two-port vector network analyzer
« on: December 21, 2018, 04:29:03 AM »
You can find the git project at

https://github.com/profdc9/VNA

What is a vector network analyzer? It is a tool for measuring impedance, reflection, and transmission.  The one I designed works on one port from about 30 kHz to 470 MHz, and on two ports from about 300 kHz to 470 MHz (though I have not tested transmission that much this high).

How does it work?  You connect a short, open, and 50 ohm load to the transmit port to calibrate the reflection, and then connect the transmit to to receive port to calibrate the transmission.  Then you can connect a load to either the transmit port, or from the transmit to the receive port, and get its impedance or transmission/reflection characteristics.

What can this do for coilers?  You could connect it to your coil and directly measure the impedance of your coil.  For example, if your system has multiple resonances, you can sweep them with very fine frequency resolution and see the upper and lower frequencies.  You can also directly measure the "Q" of your coil for example.  Basically it can measure impedances between about 2 and 10000 ohms at RF frequencies.

Does it cost a lot?  Probably about $40 in parts or so to build.  I tried to make it as easy as possible, it uses mostly through-hole parts but one surface mount part.  It even has a touchscreen.  I designed it mostly for ham radio use at HF/VHF, but it should be useful down to 30 kHz.  To be more usable at lower frequencies, it is best to put a slightly bigger common-mode choke onto it, which will make the minimum frequency lower at the expense at somewhat reducing the maximum frequency because of parasitic effects between the turns in the choke.  But even with a bigger choke it should be usable at least to 200 MHz I think.  Here's some pictures of it in action.







You can also hook it to USB and looks like a USB serial device and you can type commands at it to get data off of it.

Anyways, I know this is not high voltage, but it could be useful for high frequency circuits in high voltage.

Dan


Offline Hydron

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Re: I designed and built a two-port vector network analyzer
« Reply #1 on: December 21, 2018, 11:12:20 AM »
Woah, nice work! Will have a look at the design when i get a chance. I've already got a low frequency VNA (aka FRA) and it's super handy, including for measuring tesla coil bits.

Offline Mads Barnkob

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Re: I designed and built a two-port vector network analyzer
« Reply #2 on: December 24, 2018, 11:42:58 AM »
Thank you very much for sharing Dan, you got such a pace at new designs and implementation of existing ideas to fit the Tesla coil and high voltage branch.

I would very much like to give this a go as I only have a special audio dedicated spectrum analyzer and the spectrum analyzer functionality on the Rigol DS1054z is very rough. So a 30 kHz to ~500MHz would make a nice addition to the instrument shelf. With a simple selector switch different chokes could be selected between for different measurement ranges.

Did you try to make some measurements with a simulated load on the secondary coil? If you add a piece of thin copper wire with 6 smaller branches on it, that in size corresponds to the spark output it would give and compare that with unloaded results.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline profdc9

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Re: I designed and built a two-port vector network analyzer
« Reply #3 on: December 25, 2018, 04:12:50 PM »
I haven't hooked it up to the analyzer yet to the coil, but I'll give it a try sometime.  Mostly done antennas and filters and the like, but it should be able to measure the impedance of anything you can clip leads to.

I was thinking of adding a crude spectrum analyzer and frequency counter/measurement capability to it as well.  The second port receive port could be used with a frequency swept source to perform some spectrum analysis with up to 10 kHz IF bandwidth.  The dynamic range won't be great, but it doesn't cost anything except some additional software to write.  Also it is possible to count pulses with the microcontroller so as long as I get the frequency of the LO within 10 kHz of the signal to be counted, the pulses could be counted to determine the signal frequency by adding the measured frequency and the LO.  These features will be slow but the hardware needed to support them is already on the board and is minimal.

Dan

Thank you very much for sharing Dan, you got such a pace at new designs and implementation of existing ideas to fit the Tesla coil and high voltage branch.

I would very much like to give this a go as I only have a special audio dedicated spectrum analyzer and the spectrum analyzer functionality on the Rigol DS1054z is very rough. So a 30 kHz to ~500MHz would make a nice addition to the instrument shelf. With a simple selector switch different chokes could be selected between for different measurement ranges.

Did you try to make some measurements with a simulated load on the secondary coil? If you add a piece of thin copper wire with 6 smaller branches on it, that in size corresponds to the spark output it would give and compare that with unloaded results.

Offline profdc9

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Re: I designed and built a two-port vector network analyzer
« Reply #4 on: January 01, 2019, 03:48:21 PM »
Hello, and happy new year!

I laid out a version of the board that is mostly SMT 0805 for those who prefer that.  You can find it at

http://github.com/profdc9/VNA

A picture of it is below.

As a note, I was thinking about how to make the choke more broadband.  I think it could be done if the choke was a 50 ohm coax such as RG-174 or RG-58 wrapped 5-10 times around a high permeability MnZn ferrite like would be used for a GDT.    I think such a choke could probably work all the way from 30 kHz to 500 MHz.  The only disadvantage is that the impedance range of the analyzer might be reduced a little on the high end because of the capacitance of the coax adding a shunt reactance, so keeping the coax as short as possible is important.  I am not sure how to add a switchable capability unless I put a jumper block on the board to switch as the issue is that the parasitics a toggle switch would introduce would likely reduce the range of the impedance analyzer itself.  It's very tricky to get something to work over many decades of frequency.

Dan



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Re: I designed and built a two-port vector network analyzer
« Reply #4 on: January 01, 2019, 03:48:21 PM »

 


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